A temperature regulating system utilizes changing energy price information to alter the system operation. A performance index value is determined that indicates a balance between a user's preference for maintaining a setpoint temperature and a need for maintaining energy cost savings. The performance index value is related to the current energy price and the system automatically responds to rising energy prices by reducing system operation in a manner that provides cost savings according to a user's specified preference for weighting temperature comfort levels compared to energy cost savings. According to the described example, an optimal control solution is used to determine the performance index value.